1. Field of the Invention
The invention relates to a hub unit.
2. Description of the Related Art
A hub unit is used to rotatably support wheels of an automobile on a suspension system. The hub unit is includes an inner member, such as a hub wheel, in which an inner ring raceway is formed on its outer peripheral face and an outer member, such as an outer ring, in which an outer ring raceway is formed on its inner peripheral face, and a plurality of rolling elements provided between the inner ring raceway and the outer ring raceway. The hub wheel includes a flange portion on which, for example, a wheel and a brake disc may be fitted.
The inner ring raceway and the outer ring raceway repeatedly receive loads from the plurality of the rolling elements through long-time use and undergo rolling fatigue. Rolling fatigue causes the inner ring raceway and the outer ring raceway to flake and brings the life of a bearing to an end. Thus, to prolong the life of the bearing, there is proposed a hub unit having an induction-hardened layer formed on an inner ring raceway of a hub wheel provided with a flange portion (e.g., see Japanese Patent Application Publication No. 60-141827 (JP-A-60-141827)).
However, in the hub unit described in Japanese Patent Application Publication No. 60-141827 (JP-A-60-141827), the depth of the hardened layer tends to be increased in order to ensure appropriate hardness at a predetermined depth of the hardened layer. For this reason, the time required for induction hardening is long and hence constitutes a factor in a rise in product cost. To prevent a rise in product cost, it is appropriate to minimize the depth of the hardened layer. However, if the hardened layer is too shallow, the raceways are susceptible to flaking and the life of the bearing is shortened. That is, it has been very difficult to achieve a reduction in production cost while maintaining the sufficient life of the bearing.